The distinction between the a priori and the a posteriori has always been an epistemological one; that is to say, it has always had something to do with knowledge. The terms a priori and a posteriori are Scholastic terms that have their origin in certain ideas of Aristotle; but their use has been considerably extended in the course of history, and their present use stems from the meaning given to them by Immanuel Kant. The terms literally mean "from what is prior" and "from what is posterior." According to Aristotle, A is prior to B in nature if and only if B could not exist without A; A is prior to B in knowledge if and only if we cannot know B without knowing A. It is possible for these two senses of "prior" to have an application in common...